In electric machines, electrical currents (bearing currents) can occur in bearings which significantly reduce the service life of said bearings. Bearing currents are electrical currents which occur in antifriction bearings or sliding bearings of electric machines.
They are caused by electrical voltages (bearing voltages) which are produced on account of electrical or magnetic stray fields inside the machine or as a result of interference currents which, originating externally, flow by way of the machine. As soon as the bearing voltage exceeds the breakdown voltage (fretting voltage) of the lubricating film, the current flow takes place.
The negative effects of bearing currents are for example                fat burning (reduction in the residual lubricating capacity)        crater formation in the track and the rolling elements        and in an extreme case: ripple formation in the tracks. The ripples are oriented perpendicular to the track.        
These bearing currents are a phenomenon which has been known for decades and lead to a considerable demand on resources on the part of users or to high warranty-related costs for manufacturers. There is therefore a great interest in a measuring method or in sensors which can measure bearing currents and evaluate them in a meaningful manner.
Bearing currents on electric machines, in particular in regard to operation with power electronics, can reduce the service life of the motor bearings manyfold. According to the current prior art, bearings damaged by electrical bearing currents are noticed and replaced only when they become conspicuous, e.g. as a result of noise development or burnt bearing grease. This often leads to installation downtimes resulting in enormous costs.
A major problem in regard to operation of the bearing is therefore to recognize the anticipated point in time at which failure will occur and thus determine the optimum point in time for bearing replacement. In the case of too early a reaction this results in unnecessarily high maintenance costs, while too late a reaction means installation downtime costs for the user.
The diagnosis of the cause and assessment of solutions are dealt with at the present time on the basis of short-term bearing current measurements or vibration analyses. The validity of said measurements is limited by taking into consideration individual measurements from a period of time typically limited temporally to a few days and moreover originating from different specialists. Changes in influencing factors such as the grounding system or faults in the grounding system which occur before or after the measurements cannot be determined herewith, for example.
Previous test beds for bearing currents have hardly produced any findings relating to the prevention of damage resulting from bearing currents on account of their focus on electrical variables or vibration analyses. The same also applies in the case of measurements taken in the field. A correlation between electrical measurement and vibration measurement is not possible due to the fact that no time stamp can be recorded. Time differences are caused by the different measurement systems.
Considerable costs are often associated with remedial measures against bearing currents and the bearing damage caused thereby and it is also only possible with difficulty to evaluate the extent to which said measures will be adequate. In the past, high-cost measures have in part nevertheless not yielded the desired result.
The object of the invention is to specify a solution for the aforementioned problems. A measuring method and a device should be specified, said method allowing the bearing currents to be better evaluated in regard to potential damage to the affected bearing. Furthermore, a method and a device should be specified, said method being suitable for analyzing the cause of a damaging bearing current.
This object is achieved by a method for the early detection of the development of damage in a bearing caused by the flow of a bearing current with the following steps: measuring during ongoing bearing operation at least one variable representative of a bearing current amplitude of a bearing current, evaluating at least one long-term measurement of the at least one measured variable, transforming results from the at least one long-term measurement into a histogram which displays a frequency of occurrence of bearing currents as a function of the bearing current amplitude, and evaluating the measurement results by comparing patterns of histograms.
This object is furthermore achieved by a device for early detection of developing damage in a bearing caused by flow of a bearing current, with the device including an evaluation unit for evaluating at least one long-term measurement of at least one variable representative of a bearing current amplitude of a bearing current measured during ongoing bearing operation, means for transforming results from the at least one long-term measurement into a histogram which displays a frequency of occurrence of bearing currents as a function of the bearing current amplitude, and means for evaluating the measurement results by comparing patterns of histograms.
The device for the early detection of the development of damage in a bearing caused by the flow of a bearing current, comprises means for evaluating at least one long-term measurement of a measured variable which is characteristic of the occurrence of bearing currents during the bearing operation according to the bearing current amplitude, means for creating a representation of the measurement results based on the evaluation, and means for evaluating the representation by means of pattern detection.
In a first embodiment, the measurements are carried out in a bearing current test bed:
the cause of the bearing currents is ascertained by means of long-term bearing current measurements in a test bed, whereby the measuring time is at least longer than 1 ms, but measurements may also cover a period of days.
                The motor bearing is subjected to electrical and mechanical loads in a defined manner,        In addition to the electrical load (bearing current and voltage), the mechanical load and further parameters such as load distribution and duration or frequency range of the spark discharges are also recorded in a temporally coordinated manner, in the case of a bearing (roller bearing, ball bearing, antifriction bearing, sliding bearing) for example state of bearing grease, mechanical vibrations, temperature.        Measurement is carried out over an extended period of time (>1 hour, typically several days)        The tests are carried out in automated fashion and with temporal correlation.        
The aim is to work out the relationship between the measuring methods and to use the combination and the mathematical relationship of these different physical measured values to increase the reliability of the damage analysis.
In a further embodiment, a measurement is carried out on-site on installations:
As a result of using a bearing current sensor which monitors the bearing currents “online” continuously during operation and in an advantageous embodiment also logs selected operating parameters which have been ascertained by measurement means or by way of the control device, the following additional value is generated:                A defect in the grounding system of relevance to bearing currents is identified in a timely manner before any damage occurs. As a result of measuring the vibrations of the bearing, it is then possible to reliably predict a threat of damage and to carry out repair measures at a point in time which is acceptable to the customer in terms of cost optimization.        
The need for the introduction or monitoring of a bearing insulation can thus likewise be recognized. The measurement of bearing current and voltage as well as of the ground variables also leads to the detection of a defect. Examples of such a defect are a worn grounding brush or a damaged filter element.
A defect in the grounding system of relevance to bearing currents or a damaging change in the grounding system from the bearing current perspective is identified in a timely manner before any damage occurs. A threat of damage can then be predicted and repair measures carried out at a point in time which is acceptable to the customer.
On the basis of the operating parameters and the damage to the bearings, unfavorable combinations of bearing types and bearing parameters, of mechanical and electrical loads, can be recognized and avoided for further projects.
By means of a special evaluation of a representation in the form of a histogram (alternatively also bar chart) it is possible to differentiate different types of bearing currents. A knowledge of the type of the bearing currents enables targeted cost-effective remedial action.
Bearing Current Test Bed
As a result of the step from short-term measurements to long-term measurements (for example, using the measuring method described in publication DE 10 2005 027 670) and as a result of the combination with the measurement of mechanical variables, the inadequacy of the previous focus on primarily electrical variables and a relatively small number of operating states is avoided. The problem associated with previous measurements is the low informational value of bearing current measurements with reference to the threat of damage to the bearings. This is raised to a high informational value by the extended measuring method in which different physical measured variables are correlated with one another. As a result of the new method, relationships can now be unambiguously recognized and can be reliably evaluated using mathematical methods. Damage to machines can be prevented in this way by means of automated evaluations.
Measurement on-site on installations, e.g. within the framework of condition monitoring:                As a result of the on-site “online” analysis using new technology, changes affecting the grounding system or defective components in the grounding system which may have a negative effect on the bearing currents are recognized. According to previous methods, these factors would only be noticed as a result of noise or failure after damage to the bearings had occurred. The new measuring method enables a reliable statement which for example predicts the failure of e.g. the motor and is evaluated in a condition monitoring system in such a manner that a repair measure can be scheduled into the maintenance cycles. Trend analyses can also be performed which permit the detection of a deterioration in the system over the lifetime thereof and thereby enable a calculation of the potential point in time of a failure. This has positive effects on costs or availability of installations.        
On the basis of histograms of the measured bearing currents it is possible to differentiate the bearing current types: EDM, circular currents and rotor ground currents. In this situation, the evaluation takes place with a knowledge of the typical histogram distributions for different bearing current types.
Both the bearing current test bed and also the online diagnosis are based on the fact that a bearing current or bearing voltage sensor is permanently installed on the motor. Further sensors for registering further electrical or non-electrical variables, e.g. vibrations, are possible.
During inverter operation, current sensors and/or voltage sensors are for example often used for the motor phases and sometimes temperature sensors in the motor winding.
In the bearing current test bed, mechanical variables such as load and load distribution in the bearing can likewise be available or selectable. Rotational speed, torque and further variables can be present in the motor control. These measured values are taken into consideration in the bearing current evaluation.
The bearing current evaluation can be performed as an independent component or integrated into the motor control.
Even without motor control—e.g. in the case of operation without frequency converter—the measurement can be carried out in accordance with the described principle. In this case, the bearing current evaluation unit communicates directly with the user interface. The user interface can also be integrated into the evaluation unit.
Should the bearing currents lie beneath a certain threshold at startup time when taking into consideration further mechanical or other parameters, the startup has been carried out successfully from the “bearing current perspective”. An indication to this effect is given to the user. This indication in the user interface can be initiated indirectly by way of the motor control or directly by a bearing current evaluation unit.
If the threshold is exceeded, the startup operator can be informed in order that remedial action can be taken.
A remote diagnostic facility can similarly be integrated into the concept. To this end, the data is to be transmitted by wireless, wired, glass fiber or some other communication path. In a particularly advantageous embodiment, the user can change the type of the evaluation by means of control commands in order to refine the analyses. These are e.g. measuring range changeover, measuring filter changeover, number of measurements per second, changing the evaluation parameters such as histogram interval width in the case of statistical evaluation. An indication does not need to be restricted to a Yes/No statement. Multi-level statements right through to graphical statistical evaluations are also feasible.